To apply one of these wallpapers to your Vita's start screen, just open one of the images on the device, tap and hold on the picture, and choose 'save' to store it in your photo bank.

Then, open the 'Photos' application, navigate your way to the wallpaper you just uploaded, and tap the button that resembles a quarter circle with three dots in the centre. This opens up a menu. Choose 'Use as Start Screen'. Et voilà.